The present invention relates to a process and a device for determining the surface friction coefficients of bodies, in particular of catheters.
Medical catheters are subject to various surface treatments to, i.a., reduce the surface friction of the catheter in relation to the surrounding tissue in order to permit easier and painless insertion of the catheter, for instance into bloodstream in the case of heart catheters or into the urethra in the case of urinary catheters and to prevent possible injury to the tissue.
Catheters are often made of various, soft polymer materials, such as for example silicon rubber or polyurethane, which partially undergo different surface treatment. The purpose of this surface treatment is to smoothen and to hydrophilize in order to minimize surface friction as well as to reduce adhesion of proteins (bacteria). Strong hydrophilization of the surface decreases reciprocal energy in aqueous solutions and in that way the adhesion factor of frictional force.
Coats of different hydrogels, which feel slippery after submersion in water, have proven to be especially low-frictional. However, hitherto there is no standardized method of quantifying this xe2x80x9cslipperinessxe2x80x9d in the form of a surface friction coefficient so that presently it is not possible to quantitatively evaluate a reduction in friction due to a specific method of treating the surface.
Various attempts at measuring surface friction coefficients are already known in the state of the art. However, the measurement results determined with these known methods are not only dependent on the surface friction but also on the configuration and the consistency of the respective measured catheter and, therefore, cannot be compared with each other.
For example, many in vivo field tests were conducted for urinary catheters to determine the surface friction in the urethra of rabbits as well as humans (see Nickel, J. C. et al., xe2x80x9cIn Vivo Coefficient of Kinetic Friction: Study of Urinary Catheter Biocompatabilityxe2x80x9d, Urology XXIX (5), pp. 501-503 (1987); Khoury, A. E. et al., xe2x80x9cDetermination of the Coefficient of Kinetic Friction of Urinary Catheter Materialsxe2x80x9d, The Journal of Urology 145, pp. 610-612 (1991); Tomita, N. et al., xe2x80x9cBiomaterials Lubricated for Minimum Frictional Resistancexe2x80x9d, Journal of Applied Biomaterials 5, pp. 175-181 (1994)).
These tests are very realistic, but are unsuitable as a standard test method for catheter coatings, because particularly the muscle tone of the urethra of the individuals participating in biological tests differ. Consequently, no defined surface pressure can be applied to the catheter surface. As a result, the measured friction coefficients of a certain catheter on various individuals differ greatly.
Various attempts are also known of laboratory test systems. Two reports describe a test system in which two severed sections of the to-be-tested catheter are attached to a glass plate and are then loaded with a plane block of a defined weight coated with either collagen gel or hydrogel. A friction coefficient is determined by measuring the force needed to pull the block over the catheter (see Graiver, D. et al., xe2x80x9cSurface Morphology and Friction Coefficient of Various Types of Foley Cathetersxe2x80x9d, Biomaterials 14(6), pp. 465-469 (1993)) or by measuring the minimal oblique angle of the glass plate that is required to generate a slide movement of the block (see Nagaoka, S. et al., xe2x80x9cLow-friction Hydrophilic Surface for Medical Devicesxe2x80x9d, Biomaterials 11, pp. 419-424 (1990)). Contrary to real systems, counterform active area pairing occurs in these test systems, i.e. no full surface contact between the catheter and the block. Furthermore, surface pressure cannot be defined, because the contact surface varies depending on the softness and configuration of the tested catheter despite an unchanged weight load. Therefore, a normed friction coefficient cannot be determined with this method.
In two other methods (cf. Uyama, Y. et al., xe2x80x9cLow-frictional Catheter Materials by Photo-Induced Graft Polymerizationxe2x80x9d, Biomaterials 12, pp. 71-75 (1991)), the catheter is pulled through a bent PVC tube and the force required therefor is measured or a silicon rubber disc loaded at a defined weight is pulled over the catheter.
In both cases, there is largely conform active surface pairing, i.e. the areas of the catheter and the PVC tube respectively the silicon rubber disc are in surface contact. In the first instance, however, the normal force on the catheter cannot be defined and the size of the contact surface also varies, because the catheter deforms when pulled through the rigid tube. In the second case, the normal force varies over the contact surface and the contact surface itself also. Thus, the surface pressure cannot be precisely determined for different sizes of catheters. In both methods, an unsuited material is selected in addition for the counter body, because neither PVC nor silicon rubber is hydrophilic. Therefore, there is no similarity to real conditions when using catheters in the human or animal body.
Another attempt at determining the surface friction coefficients is known from MARMIERI, G. et al., xe2x80x9cEvaluation of Slipperiness of Catheter Surfacesxe2x80x9d, Journal of Biomedical Materials Research (Applied Biomaterials) 33, pp. 29-33 (1996). In the test method presented there, the catheter is pulled through a block of hardened agar. The time required to pull a piece of catheter out of the agar by means of a defined load of weight is set as the measure of the slipperiness of the catheter.
However, no defined surface pressure on the catheter can be given with this test system. Therefore, its does not represent a standardized test method.
Thus, none of the processes or devices known from the state of the art permit standardized determination of the surface friction coefficient of catheters. The known processes yield measuring results that are not only dependent on the surface friction but also on the configuration and the consistency of the respectively measured catheter.
Therefore, the object of the present invention is to provide a standardized process and a device for determining the surface friction coefficients of bodies, in particular of catheters, whose measuring results are reproducible and only dependent on the surface friction and are independent of the configuration and the consistency of the material of the body itself.
This object is achieved with a process and a device according to claims 1 and 9. Advantageous embodiments are the subject matter of the subclaims.
In the invented process, the to-be-examined body, for example a catheter, is pulled with a defined velocity through a gel-like, soft, viscoelastic substance and the friction force required therefor is measured. In this process, a defined pressure is applied to this viscoelastic substance, the defined pressure spreading in the entire substance due to the viscoelastic material properties. As the catheter is enclosed in the substance for a specific length l( reciprocal action length), this pressure acts particularly also on the surface of the catheter. Thus a defined surface normal force is applied to the catheter surface. In this case, the entire charged area of the catheter is A=l*xcfx80*d, with d standing for the diameter of the catheter.
When measuring catheters of different diameters d, the invented process permits holding the entire area A constant by adapting the reciprocal action length l, on which the catheter comes into contact with the viscoelastic substance, conversely proportional to the diameter of the catheter: l=A/(xcfx80*d). This can be achieved by adapting the size of the viscoelastic substance and the size of the container to receive this substance especially to each catheter diameter.
Measuring the force required to pull the catheter through the viscoelastic substance permits, with a given, known normal force, determining the surface friction coefficient of the catheter.
Of course, alternatively the viscoelastic substance can also be moved relative to a stationary catheter at a defined velocity.
The surface pressure on every surface element of the catheter surface can be exactly defined and kept constant by applying a presettable, defined pressure to the substance. Holding the surface to which pressure is applied constant permits keeping the entire acting normal force constant for every catheter diameter and for every catheter consistency. Thus, for every catheter configuration and consistency, a defined normal force exerted on a defined area of the catheter surface and the friction force can be measured. The ratio between the measured friction force and the preset normal force yields the friction coefficient. Determination of this friction coefficient is reproducible and depends only on the surface friction. Therefore, the invented process and the device therefor permit quantitative evaluation of the reduction in friction by using various methods of treating the surface of catheters.
The application of a preset, defined pressure to the substance is preferably realized by applying a defined surface pressure to a free surface of the substance. Thus, for instance, a surface pressure of approx. 104 Pa can be applied to the free surface with the aid of a water column or by means of pumping in air or water into the container with the substance.
For a useful method of determining the surface friction coefficient of catheters, the given measurement conditions should be selected as close to real conditions as possible. Therefore, it seems useful to select a physiological saline solution with a temperature of 37xc2x0 C. as the ambient medium for measuring.
The viscoelastic substance should like biological tissue be as hydrophilic as possible and be reproducible in its mechanical and chemical properties and be producible temporally stable. Hydrophilic, synthetic gel-like materials, so-called hydrogels, are therefore more suited as viscoelastic materials for this purpose than gels of natural origin.